I. Field of the Invention
This invention relates generally to cardiac pacing apparatus and more particularly to an improved physiologically adaptive cardiac pacemaker whose rate is controlled primarily as a function of the heart's pre-ejection interval but in a manner that greatly simplifies the actual implementation of the system.
II. Discussion of the Prior Art
It is now well recognized that a cardiac pacemaker which is adaptive to a patient's physiologic or metabolic demand offers advantages over fixed rate pacing. Persons skilled in the art have addressed or considered any number of different parameters which can be sensed and used to alter pacing rate. For example, the Alcidi U.S. Pat. No. 4,009,721 purports to use a pH sensor in cooperation with a demand pacemaker for causing the pacing rate to vary as a function of blood pH. Experience has shown, however, that it is difficult to find a pH probe which would remain stable over prolonged periods when implanted in the human body. The Knudson U.S. Pat. No. 4,313,412 monitors P-wave rate and varies the ventricular stimulating rate of the pacer as a function of changes in the P-wave rate. Again, the ability to reliably measure changes in P-wave rate over prolonged periods has proven difficult and furthermore, the P-wave rate is dependent somewhat on the ventricular rate and, as such, is not truly an independent variable in an adaptive system.
The Dahl U.S. Pat. No. 4,140,132 utilizes a motion sensor (accelerometer) to monitor the physical activity of the body and to alter the pacemaker's escape interval as a function of the motor activity. However, since this sensor is sensitive to any motion, it does not discriminate between externally generated sources of vibration, such as motor vehicles, and motion associated with exercise. Also, since body motion is not proportional to workload during exercise, the rate response based on a motion sensor is a non-physiological response tending to be very abrupt.
In the Salo et al application Ser. No. 362,903, filed Mar. 29, 1982, and entitled "BIOMEDICAL METHOD AND APPARATUS FOR CONTROLLING THE ADMINISTRATION OF THERAPY TO A PATIENT IN RESPONSE TO CHANGES IS PHYSIOLOGIC DEMAND", (now U.S. Pat. No. 4,686,987, issued Aug. 18, 1987) which is assigned to applicants' assignee, there is described a pacemaker in which a technique referred to as "impedance plethysmography" is used to measure the instantaneous stroke volume, on a beat-to-beat basis in the right ventricular chamber and changes in stroke volume either absolute or with respect to a reference is used to vary the pacer's escape interval. While effective rate control can be achieved using this approach, the stroke volume is somewhat dependent upon the subject's body position which can result in a pacing rate change when changing from an upright to a prone position.
The Rickards et al U.S. Pat. No. 4,527,568 describes a pacemaker which includes a means for varying the rate of the stimulus pulse generator by measuring the QT interval which, purportedly, varies in length with the physiological demands of the body. It has been shown that the QT interval of a normal healthy heart decreases as exercise or the body's demand for blood increases. It is also known, however, that the QT interval is pacing rate dependent and that the introduction of artificial stimulating pulses causes a decrease in the QT interval as well. This can result in a positive feedback situation wherein the combination of exercise and pacing results in an inordinately high acceleration.
In a patent application Ser. No. 770,205, filed Aug. 28, 1985, by Raul Chirife and entitled "CARDIAC PACEMAKER ADAPTIVE TO PHYSIOLOGICAL REQUIREMENTS", (U.S. Pat. No. 4,719,921) there is described a pacing system in which the left ventricular pre-ejection period is measured and converted to an electrical signal with such electrical signal being used to control the rate of a cardiac pacemaker. In the Chirife application, the pre-ejection period or PEP is defined as the period beginning with the QRS complex and ending with the onset of ventricular ejection. It is explained that the PEP varies predictably in response to the release of catecholamines into the blood and with direct sympathetic nerve activity when physiologic demands increase. The system of the Chirife invention suffers, however, from a practical standpoint in that it is unduly complex and difficult to implement with state-of-the-art apparatus. This is due primarily to the fact that the implementation requires a means for sensing the onset of left ventricular ejection. An effective sensor for this event is difficult to realize, given that it must be included in the implantable pacemaker housing or can.
The present invention provides a practical implementation of a rate controlled cardiac stimulator responsive to physiologic demand. Rather than attempting to measure the time between the occurrence of a paced or naturally occurring QRS complex and the onset of blood flow from the patient's left ventricle, and converting that to a voltage or digital value for adjusting the escape interval of a pacer, a multi-electrode lead located in the right ventricle is used to sense the instantaneous impedance within that ventricle as the chamber fills and empties of blood during the cardiac cycle. The resulting impedance versus time waveform is then electronically partitioned. The segment of the impedance waveform starting with a paced or sensed QRS complex (hereinafter referred to as the systole marker) and terminating at the first positive crossing of the impedance signal average following that systole marker or at some other easily defined point is the time interval of interest. This interval is found to be inversely proportional to contractility of the heart in that catecholamines or sympathetic stimulation results in a decrease of this time interval. Also, the interval in question is not subject to appreciable change due either to postural changes or changes in the ventricular pacing rate.
In that the systole marker is an easily detected event and conventional signal processing techniques, including a zerocrossing detector or other type of level detector, for operating on the impedance waveform are readily implementable with existing electronic componentry and compatible with the requirements for implantable devices (small size and low power drain), the system of the present invention proves practical and effective.
The system of the present invention also includes a means for accommodating long-term drift in the measurement of the physiologic variable. In particular, a baseline register is provided which holds a value against which the physiologic sensor output is compared in determining the ultimate pacing rate. A further multi-bit count register is incremented, decremented or left unaltered depending upon whether the sensor output exceeds, falls below or equals the baseline value, respectively, on each cardiac cycle. Subsequently, at fixed intervals greatly in excess of the time for a cardiac cycle, the aforementioned multi-bit count register is read to determine if it contains a positive, a negative or a zero value. If found to contain a positive value, the baseline value is incremented by one. If, however, the count register contains a negative value, the baseline will be decremented by one. If the count value should be zero, the contents of the baseline register is left unchanged. In this manner, over a prolonged period, the periodic adjustments to the baseline value will put the average physiological sensor output value into this register. Thus, rate changes due to long-term shifts in the output from the physiological sensor can be avoided.